Electronic trip circuit interrupters are designed to interrupt overcurrent conditions over a wide range of ampere ratings. The current through the protected electric power circuit is continuously sensed by means of current transformers and a voltage signal is supplied to the signal processor within the electronic trip unit circuit by means of so-called "burden resistors". The size of the burden resistor accordingly sets the ampere rating of the corresponding circuit interrupter. A common electronic circuit interrupter could therefore operate over a wide range of ampere ratings by merely changing the value of the burden resistor within the electronic trip circuit. U.S. Pat. No. 4,728,914 entitled "Rating Plug Enclosure for Molded Case Circuit Breakers" describes a particular rating plug that is used to vary the ampere rating of electronic circuit interrupters over several hundred amperes. It is important to prevent an electronic circuit interrupter from being inserted within an electrical distribution circuit for which the circuit interrupter is over-rated. It is perhaps equally important not to insert a circuit interrupter within an electric power distribution circuit for which the circuit interrupter is under-rated, as so-called "nuisance-tripping" could occur. It is also important to insure that a circuit interrupter is not inserted within an electric power distribution circuit with no rating plug or burden resistor whatsoever. U.S. Pat. Nos. 3,826,951 and 4,181,906 describe arrangements whereby an improper rating plug is prevented from being inadvertently inserted within an electronic circuit interrupter as well as preventing the removal of the rating plug after the electronic circuit interrupter is installed within an electric power distribution circuit.
U.S. Pat. No. 4,754,247 describes an integrated protection unit which includes basic overcurrent protection along with selected accessory features all contained within the circuit interrupter enclosure. An accessory cover allows field-installation of the selected accessories along with the installation of the electronic trip unit rating plug. The Patent further describes means for interlocking the accessory cover to prevent removal of the accessories from the circuit interrupter once the circuit interrupter is inserted within a load center or panelboard enclosure. All of the aforementioned U.S. Patents are incorporated herein and should be reviewed for their respective teachings.
One purpose of this invention is to provide an arrangement whereby the rating plug described within aforementioned U.S. Pat. No. 4,728,914 is keyed to the proper electric circuit interrupter enclosure and is interlocked with the load center or panelboard within which the electronic circuit interrupter is installed.